1. Field of the Invention
The present invention relates to the field of vehicle frames and, in particular, to a high-strength frame suitable for motorhome construction which enables a greater interior ceiling height.
2. Description of the Related Art
Recreational vehicles (RVs) have become an increasingly popular and common means of recreation. RVs are vehicles that include a living space inside and can be either self-propelled, as in a motorhome, or can depend on another vehicle for their motive power, as in campers or trailers. RVs typically provide sleeping areas, cooking facilities, and self-contained water supplies and toilet facilities. More elaborate RVs can include refrigerator/freezer units, showers and/or bathtubs, air conditioning, heaters, built in generators and/or power inverters, televisions, VCRs, and clothes washers and dryers. RVs provide many of the amenities of a residential home while on the road away from home and are popular for this, reason. RV users will typically use the RV to travel to a recreational area and live in the RV for some period of time. It is not unusual for people, particularly retired persons, to use an RV as their primary residence. RV users often have families with children and, as their trips are often of a recreational nature, will often invite friends or family along on the trip.
It can be understood that since an RV will often be used by a large number of people and often for an extended period of time, the RV manufacturers and customers will seek as many amenities and as much interior living space as possible. A major goal of RV manufacturers and their customers is to maximize the amount of usable living space inside their RVs. However, the overall size of an RV is limited both by vehicle code regulations and by practical limitations on what is reasonable to drive and maneuver. Vehicle codes restrict the maximum height, width, and length of vehicles that may be driven on public roads. Also, as a vehicle increases in size, it becomes increasingly difficult to drive and can become physically too large to pass through locations that the driver may wish to go. In addition, as the RV gets physically larger, more fuel is required to move it, which increases the cost of operation. Accordingly, many RVs are provided with slide-out room structures which are extensible so as to increase the RV's footprint and interior living space once the vehicle comes to rest.
An additional design constraint on the construction and design of RVs is their overall weight. Since an RV is intended to be mobile, either an integral power plant is provided, as in the case of motorhomes, or a tow vehicle is used to pull or carry the RV, as in the case of trailers or campers, respectively. The engine and drive-train of any motive source has an upper design limit on the weight it is capable of moving. In addition, the chassis, suspension, wheels, and brakes of a motorhome or tow vehicle also have upper design limits as to how much weight they can safely accommodate. These weight limits are established after careful engineering analysis and the weight ratings are endorsed and enforced by responsible governmental agencies. Exceeding the established weight limits of a power-train or chassis component can lead to excessive wear and failure, unacceptable performance, and exposure to liability in case of an accident.
A particular issue with the weight of a motorhome is its distribution along a vertical axis. The distance of the vehicle's center of mass from the road surface has a dramatic effect on the handling characteristics of the vehicle. The closer the center of mass is to the road surface, the shorter the moment arm between the center of mass and the roll axis of the vehicle. The shorter the moment arm between the center of mass and the roll axis of the vehicle, the less tendency the vehicle will have to lean in turns. Leaning in turns is uncomfortable for the occupants and typically places uneven loads on the tires and suspensions, compromising turning ability. For these reasons, race cars, for example, are typically built to ride as low to the ground as possible. Motorhomes, typically being quite tall, often exhibit significant leaning in turns. Obviously, the uses and requirements for an RV are quite different than those of a race car, however, within the height available in a motorhome, the weight should be concentrated as low as possible. For this reason, heavy items, such as generators, storage and holding tanks for water and fuel, and the engine are optimally placed low in the chassis.
Since RVs are mobile structures, they are typically exposed to the stresses of driving over roads that are in places quite rough. In addition, an RV will often have to travel over some distance of dirt surface to reach a camping space. Since an RV is typically used outdoors, it is exposed to the stresses of inclement weather and high winds. It can be appreciated that structural integrity is highly desired in an RV. However, the weight and size limitations previously mentioned place a limit on the strength of an RV. Accordingly, RVs are constructed to be as strong, but as light as possible.
The chassis of a motorhome is typically constructed on a preassembled steel ladder frame chassis. The chassis is a partially complete vehicle and is generally procured from a manufacturer such as FREIGHTLINER™ or FORD MOTOR COMPANY™. The chassis typically consists of two parallel frame rails extending the length of the chassis and interconnected with several perpendicular cross-braces to form a ladder frame. An engine, transmission, and fuel tank(s) are generally placed between the frame rails near one end. Suspension, steering, brake, and road wheel assemblies are attached outboard of the frame rails.
The coach bodywork, which provides and encloses the living space of the motorhome, is typically made from a laminate that can include light gauge sheet metal, plywood, vinyl, and insulation. The laminate is built to be strong, lightweight, weather resistant, and durable. The coach bodywork may also include a supporting framework. The floor of the coach typically rests indirectly on the chassis frame and the vertical walls extend upwards from the floor. The roof of the coach rests on and depends on the vertical walls of the body for structural support.
A completed motorhome may be up to 45′ long and 13′6″ high in most states. The chassis is generally on the order of 1′ high and is elevated some distance above the ground by the suspension and wheels to provide ground clearance for suspension movement and clearing obstacles in the road. The interior flooring in current art motorhomes is typically elevated a significant amount above the upper face of the chassis in order to facilitate installing ancillary equipment. In addition, many prior art motorhomes route cooling or heating air ducts adjacent the roof structure or mount air-conditioning units on the roof. Under the overall height limit previously mentioned, these structures in or on the roof intrude into the available interior height envelope and limit the usable interior vertical space. Current motorhomes typically offer interior ceiling heights of 6′9″ or less. The slide-outs in current art motorhomes do not typically provide sufficient room inside for adults to stand upright. As the slide-out area is a living space in the extended position, it can be appreciated that to be forced to stoop or crouch inside the slide-out is an inconvenience for the users of such motorhomes.
Placement of air conditioning units and/or ducting in the roof creates a further difficulty with water condensation. As air conditioning units cool air below the ambient temperature, it will be appreciated that in many conditions the temperature of the air conditioning unit and ducting carrying the cooled air will be below the ambient dew point and, thus, liquid water will condense on the cool surfaces. If these cool surfaces are located above living areas of the motorhome, as is the case with many current designs, the liquid water can be readily drawn by gravity into the interior of the motorhome. It will be appreciated that liquid water intruding into the interior of the motorhome is an annoyance at best and can damage the structural integrity of interior structures as well staining or warping interior finishings. Liquid water can also irreparably damage electronic equipment, such as laptop computers, televisions, and VCRs, such as would often be located in the interior of a motorhome.
An additional difficulty arises with motorhomes of the construction described above when the vehicle drives over rough terrain. Motorhomes are essentially rectangular and are thus susceptible to twisting deflection as opposed to a triangulated structure. Motorhomes of the construction described above are not particularly strong under torsional forces such as arise when one comer of the vehicle drops or rises compared to the others, for example when a wheel drops into a pothole or rut or the vehicle drives over a curb or speed bump at an angle. Such maneuvers “cross-up” the rectangular frame and impose twisting forces. These forces, exerted over the relatively large dimensions of a typical motorhome, can cause significant displacement in the coach. This can cause cracks to appear in the coach, jam door and window openings, dislodge interior fitments, and generally cause wear and tear on the vehicle.
From the foregoing, it can be appreciated that there is a continuing need for a stronger motorhome coach construction that also provides increased interior living space. The structure should not add an excessive amount of weight to the motorhome and should also maintain a low center of gravity to benefit vehicle handling characteristics. The design should position the air conditioning system in such a way that water that condenses out during use does not intrude into the interior of the motorhome.